These studies are concerned with the localization of the action of Ochratoxin A, produced by the fungus A. ochraceus, at the subcellular level. To this end, binding of purified ochratoxin A to mitochondria, microsomes, and nuclei is being examined. Evidence suggesting the involvement of the mycotoxin in the inhibition of mitochondrial oxidative phosphorylation will be used in conjunction with known inhibitors to determine as accurately as possible the mechanism of action of ochratoxin.